Motion sensors in smartphones have been shown to detect sound signal, for example from the rotational motions of smartphone gyroscopes or by the displacements of accelerometers. These sensors may enable continuous sound sensing, e.g., the energy-efficient accelerometer may always stay active, and turn on the energy-hungry microphone only upon detecting a keyword. While useful, these systems run pattern recognition algorithms on the features of the signals. The vocabulary is naturally limited to less than three keywords, as trained by a specific human speaker. Accordingly, while sound detection has been demonstrated, meaningful speech detection (e.g., that is meaningful to a human listener) has not been demonstrated.
Vibration motors, also called “vibra-motors,” in the relevant art are small actuators embedded in many types of phones and wearables. These actuators have been classically used to provide tactile alerts to human users, and other types of electromechanical devices are envisioned that may be capable of generating vibration signals from human speech.